In order to make wireless terminals smaller in size, it is desired to miniaturize passive devices such as filters occupying large areas in wireless portions. To this end, electromechanical filters using mechanical resonance have been proposed in place of conventional filters using electric resonance.
The size of a conventional filter using electric resonance depends on the electric length of its resonance frequency. Accordingly, there is a limit in dramatic miniaturization thereof. On the other hand, the size of a filter using mechanical resonance depends on the mass and the spring constant of a vibrator to resonate. Accordingly, it is possible to reduce the size of the filter. For example, a resonator mechanically resonating in a 1-GHz band can be made not longer than several microns though the size of the resonator depends on its shape and resonant mode.
When the resonator is put in a vacuum state, the loss in kinetic energy caused by friction with the air when the vibrator is vibrating can be reduced. Accordingly to this manner, there is an effect that a Q value can be made as high as or higher than that of a conventional filter using electric resonance.
For example, an electromechanical filter disclosed in Non-Patent Document 1 has been known as a related-art electromechanical filter using a micro-vibrator.
The electromechanical filter in this Non-Patent Document 1 is constituted by two minute dual-supported beams and lines disposed just under the two dual-supported beams through a slight gap and for inputting and outputting a high frequency signal. The two dual-supported beams are minute beams coupled with each other. The two dual-supported beams are coupled mechanically. When a high frequency signal is input to this input line, the first vibrator is excited by an electrostatic force belonging to the high frequency signal itself. In this event, when the natural frequency of the dual-supported beam coincides with the frequency of the high frequency signal, the dual-supported beam is excited strongly in a direction perpendicular to a substrate so as to begin to vibrate. Since the dual-supported beam vibrates at its natural frequency, the electrostatic capacity, that is, the impedance changes. When a DC voltage is applied, an electric current flows in accordance with the change of the impedance. In such a manner, only when the natural frequency of the vibrator coincides with the frequency of the high frequency signal, the high frequency signal is output. Thus, only a desired signal can be selected.
There has been also proposed a vibrator using a minute structure (Non-Patent Document 2). As the minute structure, multiwalled carbon nanotubes are used. The minute structure is constituted by an electrode for inputting a signal, a carbon nanotube serving as an outer shell whose opposite ends are open, and a carbon nanotube serving as an inner shell. No frictional force acts between the outer and inner shell carbon nanotubes. Accordingly, once the inner shell carbon nanotube vibrates, energy will be exchanged between the kinetic energy of the inner shell carbon nanotube and Van der Waals potential, resulting in simple harmonic motion. The vibrating direction is the longitudinal direction of the carbon nanotubes. Non-Patent Document 1: Hight Q Microelectromechanical Filters, Frank D. Bannon, IEEE Journal of solid-state circuit, Vol. 35, No. 4, April 2000
Non-Patent Document 2: Multiwalled Carbon Nanotubes as Gigaherz Oscillators, Quanshui, Physical Letters Vol/88, Number 4, 28 Jan. 2002